The present invention relates to a color image sensor for reading a color image, and more particularly to a color image sensor suitable for use as a scanner of a color printer.
A recent color printer is provided with a scanner for scanning a color original (color negative film, color positive film and the like. To reproduce a color print having proper densities and well-balanced colors, three color densities of the color original are measured at each point of the color original to control the exposure quantities of red, green and blue colors at the time of printing a color paper on the basis of the measured three color densities. A color image sensor or a solid state image sensor is used in the scanner, which may be either a three-image sensor type scanner or a single-image sensor type scanner: the former type of scanner is provided with three color image sensors, e.g., red, green and blue color image sensors for each color to be measured, and the latter type of scanner is provided with a single image sensor and a mosaic or stripe filter which is disposed at the light receiving surface of the single image sensor and has red, green and blue color transmission areas disposed alternately to each other.
The three-image sensor type scanner requires not only three color image sensors but also three sets of drivers and optical systems, thus resulting in a complicated structure and a high cost. Therefore, a single-image sensor type scanner is more advantageous in view of the cost and the space required for mounting the scanner. In the single-image sensor type scanner which has already been adopted in a color TV camera, three color signals for each pixel are mixed and collected by the scanner. However, in a color printer, it is necessary for a computer to be supplied with a color signal separated for each color in order to calculate the exposure quantity of each color. Therefore, in the case where a single-image sensor type color scanner is used for color printing, a color separation circuit of a complicated structure, which operates in synchro with reading of the color image sensor, is required between the color image sensor and the computer.
In measuring the three color densities of a color original with a single-image sensor type color scanner, shear in color registration is generated inevitably since each photoelectric conversion unit measures a different point. Consequently, a correct measurement of each pixel is not possible. This becomes a problem when the three color exposure quantities are controlled taking the hue of each pixel into consideration.
Generally speaking, color negative films made by an amateur include adequately exposed frames, over-exposed frames or under-exposed frames. Thus, a dynamic range of about 10,000 steps is necessary for measuring the density of an image of a color negative film. However, the dynamic range of a color image sensor is narrow, so that if 10,000 steps are to be covered, a change in single level due to a change in incident light must be suppressed, which makes it impossible to measure the density with high accuracy. Apart from the above, if a single frame is here considered, a dynamic range of about 100 steps can suffice for each color although the signal level differs for each color. Therefore, if the density is measured by changing the charge storage time for each color, the dynamic range is substantially widened so that a signal with less noise can be obtained.